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Education: Diesel

What is Diesel Particulate Material?

Diesel Particulate Material (“PM”, or “DPM”) is often referred to as “Diesel Particulate Matter”, “Particulate Material”, or “Particulate Matter”. PM is an aerosol comprised of complex physical and chemical structures. PM contributes to the greenhouse effect; it causes grave environmental damage, and it affects serious human health consequences. PM is responsible for the majority of the black smoke that people normally associate with diesel exhaust. PM is also a primary source of urban smog (5).

Diesel Particulate Material Composition

Diesel Particulate Matter is generally divided into three primary fractions:

  1. Solid Inorganic Fraction (SOL): is often referred to as the “dry” fraction of PM. The SOL is comprised of elemental carbons and metallic ash.
  2. Soluble Organic Fraction (SOF): is often referred to as the “wet” fraction of PM. The SOF is comprised of boiling hydrocarbons and carcinogenic dioxides.
  3. Sulfate Particulates (SO4): Sulfuric acid in diesel exhaust is derived from sulfur contained in diesel fuel. Sulfur leaves the combustion chamber primarily in th e form of sulfur dioxide (SO2) (>95%) and a small portion of sulfur trioxide (SO3). In the presence of water S03 reacts to produce sulfuric acid (S03 + H2O.= H2SO4) (6). H2SO4 is a strong, toxic mineral acid which causes acid rain, acidification of waterways, crop and property damage, harm to human health, etc.
To eliminate potential confusion, it should be noted that “PM”, or “DPM” is also often referred to as “TSP” (Total Suspended Particulate) and/or “TPM” (Total Particulate Material)

Generally, (SOL + SOF + Sulfate Particulate) = TPM (7), and TPM = PM = DPM = TSP.

Although specific values fluctuate based on variables which include: engine calibration, engine loads, fuel qualities, etc., a good generalization of PM composition is that PM is composed of 30 % “wet” material and 70% “dry” material (8,9,10).

Particle Size Classification

Particulate Material is also divided into particulate sizes. Diesel Particulates have a binomial size distribution, which includes nuclei mode particles, and larger, accumulation mode particles (11). The most common sizes used to discuss particle material are PM 10 and PM 2.5. Sources differ in their classification of smaller sized particles, i.e. particle sizes that are less than 2.5 µm in diameter. For purposes of general discussion however, particle sizes can be grouped into the following four categories:

  1. PM 10 (larger particles): are defined as less than 10 µm in aerodynamic diameter. PM 10 comprises a small percentage of the total particulate count, but it constitutes the majority of the total particulate mass.
  2. PM 2.5 (fine particles): PM 2.5 (and smaller particles categories) constitute the majority of the particle count (in most cases ≥ 90%), but account for very little of the total particulate mass (in most cases ≤ than 3%).
  3. Ultra-fine particles are those below 0.1 µm or 100 nm
  4. Nano-particles have diameters of less than 50 nm (12)
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